1. Field of the Invention
The present invention relates to a semiconductor device and a control method thereof, and more particularly, to a semiconductor device having a function of rapidly turning off, upon detection of an abnormality, an output transistor that supplies a current to a load, and a control method thereof.
2. Description of Related Art
In recent years, semiconductor devices have been used in systems having a switch function capable of driving a large current, as typified by electrical components for automobiles. Each system that drives a large current has a function of switching itself off upon detection of an overheat, an overcurrent, or the like so as to protect the system when an abnormality occurs in which a load is short-circuited and a large current flows, for example.
In the operation in which the system switches itself off upon detection of an abnormality, it is important to reduce a heat loss that occurs upon turn-off. This is because if a large heat loss occurs upon turn-off, a breakdown may occur, since it is highly possible that a large amount of heat is generated when an abnormality is detected. Accordingly, in the turn-off operation upon detection of an abnormality, it is important to rapidly turn off.
FIG. 12A is a block diagram showing a high-side output IC 30 which is provided with a GND terminal. Since the high-side output IC 30 is provided with the GND terminal, an electric charge stored in the control terminal of the output transistor can be discharged not only to an output terminal OUT but also to the GND terminal to turn off the high-side output transistor. The turn-off operation is achieved by discharging the electric charge to the output terminal OUT or to both the output terminal OUT and the GND terminal. Meanwhile, when the GND terminal is not provided, an IN terminal of the circuit serves as the GND terminal. Upon receiving an input OFF signal, the IN terminal becomes “H” level. Generally, the high-side output transistor is turned off by discharging the electric charge from the gate only to the output terminal OUT.
FIG. 13 is a diagram showing an output circuit 200 of prior art (Japanese Unexamined Patent Application Publication No. 2005-123666 (kojima)). Transistors Q1 and Q2 are N-type depletion or enhancement transistors, and a transistor Q3 is an N-type enhancement transistor. A power supply terminal VCC and an output terminal OUT are connected through an output transistor 100. Signals for controlling the transistors Q1, Q2, and Q3 are output from a state determination circuit. The output circuit 200 performs a turn-off operation as described below. The output circuit 200 activates one or more of discharge paths. The discharge paths are formed of Q1 and R1, Q2 and R2, and Q3, respectively. The gate charge of the output transistor is discharged to the output terminal OUT, thereby turning off the output transistor.
During a normal turn-off operation, the transistor Q1 is activated. During a rapid turn-off operation in an overcurrent state, only the transistor Q3 is activated, or the transistors Q1, Q2, and Q3 are simultaneously activated. During the turn-off operation in which the current is larger than that in the normal state and smaller than the overcurrent, only the transistor Q2 is activated, or both the transistor Q1 and Q2 are activated at the same time. The slew rate is adjusted by changing the paths to be activated according to the current value.
In the overcurrent state, the operation of the transistor Q3, which is an enhancement transistor, is especially important. The rapid turn-off operation is accomplished in such a manner that the transistor Q3 rapidly discharges the gate charge of the output transistor. Assume herein that a potential of the output terminal OUT is OUT; a potential of the power supply terminal VCC is VCC; an on-resistance of the transistor Q3 is Ron; a current flowing through the output transistor 100 is IL; and a gate-source voltage of the transistor Q3 is Vgs.